Many heat exchangers, particularly those having fins made of aluminum or aluminum alloy are designed to provide the maximum surface area for a heat radiating unit and a cooling unit to allow high heat-radiation efficiency or cooling efficiency in the limited space. Because of that, a gap between the fins is extremely narrow. When using these heat exchangers for cooling, moisture in the air is sometimes condensed on fin surfaces into droplets of water to adhere onto the surfaces. Such adhesion of the water droplets causes clogging between the fins and thereby increases draft resistance, thus reducing heat exchange efficiency.
In addition, the condensed water on the fin surfaces may induce corrosion of the aluminum or aluminum alloy to produce a large amount of aluminum oxide in a white powder form on the fin surfaces. Furthermore, when the fins are exposed to high temperature conditions while retaining the condensed water on the surfaces, a high-temperature and high-humidity atmosphere is produced, whereby a thick layer of white powdery aluminum oxide may be produced on the fin surfaces. The white powder of aluminum oxide deposited on the fin surfaces is scattered inside the automobile room and the like by a fan of the heat exchanger. This makes the user feel uncomfortable.
In addition, when the condensed water on the fin surfaces stays, bacteria easily grow on a surface of the aluminum or aluminum alloy, which can cause bad odor or damage the health of the user. Moreover, when an odor component of tobacco smoke or the like supplied to the heat exchanger from the inside of the automobile room or the like adheres to and accumulates on the surface of the aluminum or aluminum alloy, bad odor is released from the heat exchanger.
Accordingly, it is desirable to provide a function of maintaining hydrophilicity, high corrosion resistance, antibacterial properties, and deodorant properties to the aluminum or aluminum alloy forming the heat exchanger.
As a conventional technique to solve those problems, as in Patent Document 1, there has been proposed a method in which a conversion layer is formed on a surface of an aluminum containing metal substrate and thereon is formed a resin layer having a water-insoluble three-dimensional network structure by crosslinking reaction between a water-soluble crosslinking polymeric compound having a hydrophilic group such as a sulfonate group and a crosslinking agent, whereby the network structure retains the water-soluble polymeric compound to prevent the elution thereof. The Literature describes that the method can uniformly form, on the aluminum containing metal material, a resin coating film maintaining hydrophilicity, water resistance, and resistance to swelling by water and having excellent antibacterial properties and odor production-preventing properties over a long period of time. In this method, however, an odor component of tobacco smoke or the like supplied to the heat exchanger from the inside of the automobile room or the like adsorbs to and accumulates on the hydrophilic coating film. Thus, it has been impossible to maintain sufficient deodorant properties, as well as to sufficiently achieve corrosion resistance in atmospheres subjected to high temperature and high humidity.
In addition, as in Patent Document 2, there has been proposed a method for maximally preventing odor components of tobacco or the like from adhering to a heat exchanger made of aluminum alloy by applying also a coating of an organic hydrophilic coating film containing (A) one or more types of compounds selected from chitosan and derivatives thereof and (B) one or more types of specific organic compounds having two or more carboxyl groups per molecule such that a total solid content thereof (A)+(B) is 20% by mass or more, on at least a part of a surface of the heat exchanger made of aluminum alloy. However, the method has failed to maintain hydrophilicity over a long period of time.